1. Field of the Invention
This invention relates to a display apparatus which includes a pixel array section including a plurality of pixel circuits (hereinafter referred to also as pixels) disposed in rows and columns and each including an electro-optical element (hereinafter referred to as display element or light emitting element). More particularly, the present invention relates to a display apparatus of the active matrix type wherein a plurality of pixel circuits each including an electro-optical element whose emission light luminance varies depending upon current flowing therethrough are disposed in rows and columns and display driving in a unit of a pixel is carried out by an active element included in each of the pixel circuits.
2. Description of the Related Art
A display apparatus is available which uses, as a display element of a pixel, an electro-optical element whose emission light luminance varies depending upon a voltage applied thereto or depending upon current flowing therethrough. For example, a liquid crystal display element is a representative one of electro-optical elements whose emission light varies depending upon a voltage applied thereto. Meanwhile, an organic electroluminescence (hereinafter referred to as organic EL) element such as an organic light emitting diode (OLED) is a representative one of electro-optical elements whose emission light luminance varies depending upon current flowing therethrough. An organic EL display apparatus which uses the latter organic EL element is a selfluminous display apparatus which uses an electro-optical element, which is a selfluminous element, as a display element of a pixel.
An organic EL element includes a lower electrode, an upper electrode, and an organic thin film or organic layer disposed between the upper and lower electrodes and formed by laminating an organic hole transport layer, an organic light emitting layer and so forth. With the organic EL element, a gradation of color development is obtained by controlling the value of current flowing through the organic EL element.
Since the organic EL element can be driven with a comparatively low application voltage such as, for example, 10 V or less, it exhibits low power consumption. Further, since the organic EL element is a selfluminous element which itself emits light, the organic EL display apparatus does not require an auxiliary illuminating member such as a backlight which is required by a liquid crystal display apparatus, and therefore, reduction in weight and thickness can be achieved readily with the organic EL display apparatus. Furthermore, since the response speed of the organic EL element is very high such as, for example, approximately several μs, an afterimage does not appear upon dynamic image display. Since the organic EL element has such advantages as described above, a display apparatus of a plane selfluminous type which uses an organic EL element as an electro-optical element has been and is being developed energetically in recent years.
Incidentally, a display apparatus which uses an electro-optical element including a liquid crystal display apparatus which uses a liquid crystal display element and an organic EL display apparatus which uses an organic EL element can adopt, as a driving method, a simple or passive matrix system and an active matrix system. However, although the display apparatus of the simple matrix system is simple in structure, it has a problem that it is difficult to implement a display apparatus of a large size and a high definition.
Therefore, in recent years, a display apparatus of the active matrix system is developed energetically wherein a pixel signal to be supplied to a light emitting element in a pixel is controlled using an active element formed within a pixel, for example, an insulated gate field effect transistor, usually, a thin film transistor (TFT), as a switching transistor.
In order to cause the electro-optical element in the pixel circuit to emit light, an input image signal supplied through an image signal line is fetched into a storage capacitor or pixel capacitor provided at the gate terminal, which is a control input terminal, of a driving transistor through a switching transistor (hereinafter referred to as sampling transistor). Then, a driving signal in accordance with the fetched input image signal is supplied to the electro-optical element.
In a liquid crystal display apparatus which uses a liquid crystal display element as an electro-optical element, since the liquid crystal display element is an element of the voltage driven type, the liquid crystal display element is driven by a voltage signal itself corresponding to the input image signal fetched in the storage capacitor. In contrast, in an organic EL display apparatus which uses an element of the current driven type such as an organic EL element as an electro-optical element, a driving signal in the form of a voltage signal corresponding to the input image signal fetched in the storage capacitor is converted into a current signal by a driving transistor. Then, the driving current is supplied to the organic EL element and so forth.
In an electro-optical element of the current driven type represented by an organic EL element, where the value of driving current differs, also the emission light luminance differs. Therefore, in order to cause the electro-optical element to emit light with stable luminance, it is important to supply stable driving current to the electro-optical element. For example, driving methods for supplying driving current to the organic EL element can be roughly divided into a constant current driving method and a constant voltage driving method. Such driving methods are known and are not described specifically herein.
Since the voltage-current characteristic of the organic EL element has a steep slope, if constant voltage driving is applied, then a small dispersion of a voltage or a small dispersion of an element characteristic gives rise to a great dispersion of current and gives rise to a great luminance dispersion. Therefore, constant current driving wherein a driving transistor is used in a saturation region is used popularly. Naturally, even with constant current driving, if some current fluctuation exists, then this gives rise to a dispersion in luminance. However, if the current dispersion is small, then only small luminance dispersion occurs.
Conversely speaking, even where the constant current driving method is used, in order to make the emission light luminance of the electro-optical element invariable, it is significant for the driving signal, which is written into and stored in the storage capacitor in response to an input image signal, to be fixed. For example, in order for the emission light luminance of the organic EL element to be invariable, it is important for the driving current corresponding to the input image signal to be fixed.
However, the threshold voltage or the mobility of the active element, that is, a driving transistor, for driving the electro-optical element is dispersed by a process fluctuation. Further, a characteristic of the electro-optical element such as an organic EL element is fluctuated as time passes. If such a characteristic dispersion of a driving active element or a characteristic fluctuation of an electro-optical element exists, then this has an influence on the emission light luminance even where the constant current driving method is applied.
Therefore, in order to control the emission light luminance so as to be uniform over an entire screen of a display apparatus, various mechanisms for compensating for a luminance fluctuation arising from a characteristic fluctuation of a driving active element or an electro-optical element in each pixel circuit are investigated.
One of such mechanisms as just described is disclosed, for example, in Japanese Patent Laid-Open No. 2006-215213 (hereinafter referred to as Patent Document 1).
For example, according to the mechanism disclosed in Patent Document 1, a pixel circuit for an organic EL element is disclosed which has a threshold value correction function for making the driving current fixed even where the threshold voltage of a driving transistor suffers from a dispersion or aged deterioration, a mobility correction function for making the driving current fixed even where the mobility of the driving transistor suffers from a dispersion or aged deterioration and a bootstrap function for making the driving current fixed even where the current-voltage characteristic of an organic EL element suffers from aged deterioration.